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Functional Inorganic Materials and Devices

Multifunctional Screen-Printed TiO2 Nanoparticles Tuned by Laser Irradiation for Flexible and Scalable UV Detector and Room Temperature Ethanol Sensor Georges Dubourg, and Marko Radovic ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19976 • Publication Date (Web): 17 Jan 2019 Downloaded from http://pubs.acs.org on January 22, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Materials & Interfaces

Multifunctional Screen-Printed TiO2 Nanoparticles Tuned by Laser Irradiation for Flexible and Scalable UV Detector and Room Temperature Ethanol Sensor Georges Dubourg*, Marko Radović BioSense Institute-Research and Development Institute for Information Technologies in Biosystems, University of Novi Sad, Dr Zorana Đinđića 1, Novi Sad 21000, Serbia KEYWORDS: Laser processing, multifunctional sensors, flexible substrates, screen-printing, titanium dioxide

ABSTRACT. Recently, multifunctional devices printed on flexible substrates, with multisensing capability, have found new demand in the practical fields of application such as wearable electronics, soft robotics, interactive interfaces and electronic skin design, revealing the vital importance of precise control of the fundamental properties of metal oxide nanomaterials. In this paper, a novel low-cost and scalable processing strategy is proposed to fabricate all-printed multisensing devices with UV and gas sensing capabilities. Undertaken approach is based on the hierarchical combination of screen-printing process and laser irradiation post-treatment. The screen-printing is used for the patterning of silver interdigitated electrodes and the active layer based on anatase TiO2 nanoparticles while the laser processing is utilized to fine-tune the UV

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and ethanol sensing properties of the active layer. Different characterization techniques demonstrate that the laser fluence can be adjusted to optimize the morphology of the TiO2 film by increasing the contribution from volume porosity, in order to improve its electrical properties and enhance its UV photoresponse and ethanol sensing characteristics at room temperature. Furthermore, results of the UV and ethanol-sensing investigation show that the optimized UV and ethanol sensors have good repeatability, relatively fast response/recovery times and excellent mechanical flexibility.

1. INTRODUCTION Metal oxides (MOx) represent an important class of materials since they can be easily shaped to provide them with specific features and improved mechanical, chemical, electrical and optical properties. It’s then clearly understandable why they have attracted lot of attention in a wide range of applications, including optoelectronics, gas sensing devices

1-4

and more recently in the

development of multifunctional devices able to perform several of these tasks.

5-8.

Their

conjunction with printing technologies and flexible substrates can offer the potential for scalable and low-cost fabrication and open new opportunities for various sensing applications in wearable and flexible multisensing devices including watches, glasses, patches, bandages, food packaging and other fields that are currently difficult to reach with traditional semiconductor technology. A major challenge for the fabrication of flexible devices based on MOx materials is to lower the processing temperature in order to make it compatible with flexible substrates such as plastics, as most plastic substrates degrade at ∼150 °C. Indeed, annealing process at elevated temperature is an important step when processing MOx material since it contributes to enhancement of specific properties and thus enables integration in functional devices. Typically, this step ensures the removal of residual solvents and organic binders, improves the chemical homogeneity of the


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MOx based film, controls its stoichiometry, composition, phase structure and microstructure and improves the interfacial adhesion of the film/substrate and interparticle connection. The thermal treatment of MOx nanomaterials is generally carried out in oven or furnace at temperatures ranging from 300 to 1000 °C which are clearly incompatible with thermally sensitive substrates like cellulose, textile and polymers. Some solutions have been explored to overcome this problem including near infrared

9, 10

microwave

11, 12

and laser treatments.

13, 14

Among them,

laser sintering offers many advantages because of its capability of selective treatment and localized thermal effect, its fine spatial resolution along with the possibility to be processed in ambient atmosphere. Over the past decade, laser irradiation has been employed in conjunction with diverse MO films and nanostructures. For instance, laser treatment was used to improve the gas sensing and photocatalytic properties of nickel oxide (NiO).

15, 16

Effect of the laser

irradiation on ZnO and SnO2 was also studied to enhance their electrical and optical properties and surface morphology for transparent electrodes, sensor applications.

21-23

17, 18

field effect transistor,

19, 20

and gas

Furthermore, interaction of the laser irradiation with TiO2 was

extensively explored but with an important attention on the enhancement of its dye sensitized solar cells (DSSC) and photocatalytic performance through optimization of basic parameters of the laser treatment method 24-28 even though TiO2 has other outstanding properties. Indeed, it was identified to be appropriate material in many fields including water purification, photocatalysis,29, 30 UV and gas sensors, 31-33 dye-sensitized solar cells 34 light-emitting diodes, 35 and in biomedical engineering 36 because of its unique electrical, physical, chemical, and optical properties. Hence, to fully exploit the unique properties of TiO2 especially those related to its sensing capabilities, it is essential to investigate the effect of the laser irradiation on the sensing


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behavior of TiO2 which is a key element for its integration in flexible and wearable multifunctional devices. In this present work, we introduce a novel approach for tailoring the UV and ethanol sensing capabilities of screen-printed anatase TiO2 nanoparticles. The entire process includes two screen printing steps and low-temperature laser post-treatment, indicating the possibility of arbitrary substrates and low-cost and large-area fabrication of multisensing devices. Effects of the laser irradiation on UV and gas sensing performances on the screen-printed TiO2 nanoparticles have been studied in detail and it was found that a substantial improvement of the photoreseponse and ethanol sensitivity can be obtained by simply adjusting the laser fluence. Basic mechanisms for the observed behavior are discussed and correlated with results from surface analysis and electrical characterization of the TiO2 laser treated film. Photo and gas sensing characteristics, including repeatability, response times and mechanical stability are carefully examined in order to demonstrate the actual performance of the printed devices to be used as a flexible multisensing platforms. 2. EXPERIMENTAL PROCEDURE Multifunctional sensors were fabricated in a resistive configuration consisting of TiO2-based active layer deposited on interdigitated electrodes (IDE) previously patterned on a flexible substrate. The fabrication process of the flexible UV and ethanol sensors, made of a single common active material (anatase TiO2 nanoparticles) is shown in Figure 1. Briefly, we selected lightweight and economical PET (poly ethilene teraphtalate) substrate as a flexible building platform for the sensor design. Then, in order to preserve the attractiveness of the sensors as lowcost and potentially mass-produced devices, we developed a simple and economic technological process consisting of two screen-printing steps for the deposition of silver electrodes and paste


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with TiO2 nanomaterial. Initially, commercial silver paste (HPS-021LV, Novacentrix ) was screen-printed with a semi-automatized screen-printer (EKRA 2H screen-printer, Dornstadt, Germany) on the PET substrate to create the interdigitated electrodes (Figure 1a). After printing, the samples were backed in oven at 110°C for 15 min. Individual digit of the IDE structure is 4 mm long and 100 μm wide, and it is separated by a gap of 100 μm to the next digit as shown in Figure 1b exhibiting SEM (Scanning electron microscope) image of Ag interdigitated electrodes. The next step involved the screen-printing of the TiO2-based paste on the top of the IDE. For this purpose, a TiO2-based paste was prepared and then screen-printed in order to cover locally the surface of the electrodes (Figure 1c). The functional paste based on TiO2 nanoparticles was prepared following the previously published procedure,

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where commercial

anatase TiO2 nanopowder, PVP (Poly vinyl pyrrolidone) and terpineol were used. First, PVP from Sigma-Aldrich (2 g) was dissolved in ethanol (10 ml). Next, anatase TiO2 (1 g) nanopowder (Sigma-Aldrich, particle size < 25 nm) was dispersed in terpineol (400 µl) (Sigma-Aldrich), followed by addition of PVP solution (600 µl). Obtained suspensions were treated with ultrasonic horn for 10 min respectively. After screen-printing, the sample with the resulting patterned TiO2 based-paste were dried at ambient temperature for 2 days. Afterwards, a laser processing step was employed in order to control precisely the properties of the screen-printed TiO2 films in a large -scale manner (Figure 1d). The laser treatment of printed films was carried out by using a diode-pumped Nd:YAG laser cutter Rofin-Sinar Power Line D– 100, operating in the NIR range at 1064 nm. The length of the line scanned by the laser was designed to be 5 mm, with line intervals of 10 µm. The laser beam was precisely focused on the surface of the screen-printed film and the laser pulse was set with a frequency of 65 kHz and a speed of displacement of 500 mm/s in order to obtain sufficient pulse overlapping. Afterwards,


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the laser fluence was varied by adjusting the laser pump current. Basically, three samples were treated with input current values of 27 A, 28 A and 29 A corresponding to laser fluences of 0.17 J cm-2, 0.19 J cm-2 and 0.21 J cm-2, respectively, and one sample was kept untreated to be used as a reference. Figure 1e shows a matrix of laser treated films made of TiO2 nanoparticles, providing a clear evidence that the developed process is convenient for the large-scale fabrication of functional sensors.

a)

Squeegee moving direction

Screen Screen-printing of silver electrodes

b) PET substrate

Silver electrodes

150 µm

c)

Screen-printing of TiO2 paste

e) d)

Laser sintering

Figure 1. Schematic representation of multisensor platform design. a), c) and d) Process sequence of the flexible devices, b) SEM image of the silver electrodes, e) Optical image of 3 × 5 sintered device matrix fabricated on PET substrate.


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Micro-Raman spectroscopy measurements were performed in the 1000 cm-1 to 3000 cm-1 range, on the Jobin Yvon T64000 spectrometer, equipped with a nitrogen-cooled CCD detector and the solid state Nd:YAG laser, operating at λ = 532 nm The morphology of the screen-printed TiO2 film were examined by scanning electron microscope (HITACHI TM3030) and atomic force microscopy (AFM). Images from AFM were collected with an NTEGRA prima microscope, operating in semi-contact mode. The thickness profiles were measured from SEM images with the software 3D image Viewer (HITACHI) and roughness characteristics of the film were obtained from the multi profile analyses provided in the supporting information (Figure S1, Table S1) and measured with the same software. Profile analysis of the AFM measurements was performed from the 2D AFM images provided in supporting information (Figure S-2) using Gwyddion software. The electrical properties of the sensors were measured by using a Yokogawa–Hewlett-Packard semiconductor probe analyzer and optical transmission measurements with a UV-VIS spectrometer (DSH-L6-L6S). The photo activation of the devices was carried out with a 365 nm LED in a faraday cage and characterized with the same semiconductor probe analyzer. For the ethanol sensing characterization, the contact pads of the printed devices were connected to wires, fixed on a plastic holder and placed on the top of a closed 15 cl plastic chamber composed of an inlet for ethanol solution injection, controllable N2 flow system, evaporator and fan for ethanol vapor circulation. The sensing operation was performed at 25°C, and the ethanol gas sensing characterization was monitoring using the semiconductor probe analyzer.


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3. RESULT AND DISCUSSION 3.1. Structural characterization Figure 2a shows optical images of untreated and samples sintered at laser fluences of 0.17 J cm2

, 0.19 J cm-2 and 0.21 J cm-2. It can be seen that laser sintering introduces a change in the

apparent color of printed films, from white for the untreated TiO2 to grey for the samples treated with laser fluence of 0.17 J cm-2 and as the laser fluence increased the samples become black. Observed changes in contrast can originate from carbonization of organic binders present in the paste, during the laser treatment or from micro-scale reorganization of the surface roughness and porosity. In order to gain better insight into chemical composition of the laser treated film and to directly monitor changes in carbon content, Raman spectroscopy measurements were performed in the spectral range where organic (carbohydrate) molecules have distinct fingerprint modes, before and after laser treatment. From the analysis of obtained results (Figure 2b) it can be clearly seen that laser treatment induces dramatic decrease in the carbonyl stretching peak at 1605 cm-1

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which is characteristic for PVP polymer, providing clear evidence of complete

degradation and removal of PVP component (chemical structure given in the figure 2b)). In the case of C – H bond at 2935 cm-1, we can see very weak peak in spectrum (Figure 2 c) even after treatment at highest laser fluence. Such findings point out that after laser sintering there are some traces of organic components in investigated film, which probably originate from terpineol component that was used in the paste matrix. Most importantly is to highlight the fact that characteristic peak of graphite or similar compounds were not detected indicating that darkening of the samples does not originate from the formation of carbon allotrope such as graphite but, probably from the micro-scale reorganization of the surface roughness and porosity that strongly diffuses the incident light. In addition, it can be noticed from the EDX characterization shown in


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Figure S3 that the Ti and O signatures are present in the untreated and laser treated samples which confirms that the laser irradiation does not change the chemical composition of the TiO2 nanoparticles. a)

b)

c)

c)

Untreated 0.17 J cm-2

0.19 J cm-2 0.21 J cm-2

Figure 2. a) Optical images of untreated and laser sintered samples.b) Raman spectra of the untreated sample and samples treated at 0.17 and 0.21 J cm-2 in the spectral range 1000 and 1800 cm-2. c) Raman spectra of the untreated sample and samples treated at 0.17 and 0.21 J cm-2 in the spectral range 2800 and 3200 cm-2 Therefore, additional investigations were performed with SEM measurement to evaluate the change of the film morphology in the laser sintered samples. Figure 3a exhibits top and sideview images of the TiO2 film before and after laser treatment at various laser fluences which clearly demonstrate that increase in laser fluence induces significant modifications of the film morphology. From SEM image of untreated film, it can be established that micron sized particles, in a form of clusters, constitute the surface layer. These agglomerates are formed from individual nanoparticles held together by the dried organic additives. When irradiated at 0.17 W cm-2 the film roughness increases, exhibiting bigger pores due to the formation of melted droplets. These droplets are formed during the breaking of the large agglomerates accomplished by laser thermal evaporation of organic components, but the investigated film still lacks better interconnection between the individual TiO2 particles. With further increase of the laser fluence,


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laser treatment leads to sintering of the anatase nanoparticles and the printed film becomes denser due to the formation of necking between individual nanoparticles introducing better mutual connectivity. Figure 3a reveals that the film sintered at 0.19 J cm-2 still has a rough film, but the formation of solid layers becomes dominant, due to the complete release of the organic components and the sintering effect. When irradiated at 0.21 J cm-2, the screen-printed film is transformed into solidified, uniform and dense ceramic material. These results are confirmed by the 3D AFM images of the untreated sample and film treated with 21 J/cm-2 laser fluence exhibited in Figure 3b. Indeed, in this figure, it can be clearly observed that the surface of untreated sample consists of agglomerated micron-sized particles, whereas surface of laser treated films exhibits finer surface structure with particles in nanometer range, in agreement with SEM measurements. Direct comparison of AFM images offers clear evidence that laser treatment induces breaking of large agglomerates into nanoparticles and provides a powerful tool for fine tuning of surface corrugation. In Figure 3c are compared typical line profiles from AFM measurements, taken along the A-A’ direction as denoted in corresponding 3D images. Observed behavior points out that variations in the line profile of sintered sample are less pronounced in comparison to the untreated sample, suggesting that laser treated surface has improved surface ordering, lower roughness and much finer morphology. Figure 3d shows the root mean square roughness Rq as function of the laser fluence obtained from multi profile analyses. It can be noticed that the roughness of the TiO2 film is increased when the film is treated at 0.17 J cm-2, and then decreased when the surface is treated at higher laser fluences due to the densification of the TiO2 film, confirming results deducted from SEM and AFM images.


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Figure 3e shows the thickness profile of the investigated samples. Presented results demonstrate that the film thickness is reduced from 20 to 13.00, 10 and 6 μm after irradiation at 0.17, 0.19 and 0.21 J cm-2, respectively which can be directly correlated to the removal of organic binder components with laser radiation and the densification of the screen-printed TiO2 film during sintering process. 1

c) c)

a)

Untreated -2 0.21 J cm

0.8

Height (µm)

13.8 µm 22.3 µm

0.6 0.4 0.2

20

Film thickness (μm)

10

6.3 µm

9.6 µm

0.17J/cm 0.19 J/cm

0

0.17J/cm

0

0.19 J/cm

2

0.21 J/cm

4

6

8

A-A’ cut (µm)

0.21 J/cm

d) 2,5

10

2 1,5

0 0

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Untreated

Untreated

Rq (µm)

20

0.17 J cm -2

Untreated


200 400 Length (μm)


600

600 1

0,5

0

0,17

0,19

0,21

Laser fluence J cm-2 0.19 J cm -2

0.21 J cm -2

e) 20

Untreated 2

0.17J/cm 0.17 J cm-2

b) A’ A’

A

A 2

0 µm

2

0.21JJ/cm 0.21 cm-2

10

A

Untreated

0.19 cm-22 0.19JJ/cm


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10 µm

0.21 J cm-2

10 µm

0 0 µm

0


600

Figure 3. a) SEM images of untreated and laser treated screen-printed TiO2 films, inset: side view of untreated and treated samples. b) AFM characterization of the untreated sample and sample traited at 0.21 J cm-2. c) Thickness profile from AFM analysis of the untreated sample and sample traited at 0.21 J cm-2. d) Root mean square roughness of the investigated films as function of the laser fluence. e) Thickness profile dependence on laser fluence for the different TiO2 films.


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3.2. UV detection performance UV sensors are employed today in a wide range of applications including healthcare, space technology and environmental monitoring. Those applications require often large-scale and lowcost fabrication process and certain flexibibility to allow their integration into wearable and flexible devices which are capable of working under inherent bending. Today, this could be achieved through printing technology; hence several studies regarding printed photodetectors on flexible substrates based on various metal oxide materials have been reported.

39,40

However,

there is still a lack in scalable and routine process to allow their integration into a large and flexible surface. The process proposed in our work is fast, scalable, low cost and does not require high tempareture steps which is fully compatible with flexible substrate technologies. As described previuosly, our method consists in only two screen-printing steps and a post-treatment step by laser sintering to optimize the UV photosensitivity of the TiO2 screen printing film. To evalute the potential of the resulting samples to be used as UV detector, DC (Direct current) electrical charaterization was performed for the untreated and samples sintered at 0.17 J cm-2 , 0.19 J cm-2 and 0.21 J cm-2 laser fluences. Figure 4a shows I(V) curves of the investigated sensors, measured in the − 5 to 5 V range. All measured curves exhibit typical semiconductor response, with an evident deviation from linear current–voltage behavior. Figure 4a clearly reveals that increase in laser fluence induces significant increase in the current values, thus improving the conductivity of the printed TiO2 film. Indeed, for untreated sample, the profile of I(V) curve corresponds to the material with poor conductivity (1.36 nA at 5 V), whereas the I(V) curve of the sample treated with 0.21 J cm-2 has three order of magnitude higher current (341μA at 5 V). For porous nanomaterials, where nanoparticles are packed together, DC transport properties of the film are generally governed by the grain-boundary resistance, since


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the resistance at these contacts is much higher than the resistance across single nanoparticle. During sintering process, most of these grain boundaries vanish as the nanoparticles form necklike structures and the grains connect together. In this case, the grain conductivity becomes dominant

41

leading to observed increase in the current values. Another contribution to the DC

conductivity comes from formation of oxygen vacancies in TiO2 nanoparticles with laser treatment, since these defects introduce additional electronic states inside the band gap (donor impurities). 37 Afterwards, in order to define the best sintering process for the device to be used as UV detector, photocurrent measurements were performed for the fabricated TiO2 printed devices before and after laser treatment at 0.17, 0.19  and 0,21 J cm-2. To highlight the UV detection capabilities of the devices, all samples were studied by measuring the current–voltage (I–V) relationships at 365 nm wavelength illumination at 20 mW cm-2 light intensity and plotted in Figure 4b together with the dark current for comparison. Figure 4b clearly demonstrates that significant increase in the current is observed under UV light illumination for the untreated and samples treated at 0.17 and 0.19 J cm-2 while no significant rise of current can be noticed for the sample treated at 0.21 J cm2

. This is confirmed in Figure 4c showing the photocurrent-to-dark current ratio called the photo

response and the dark current of the TiO2 film as function of the laser fluence. The photo response is improved as the the laser fluence is raised to 0.19 J cm-2 and subsequently deteriorated for higher laser fluence, while the dark current is still rising as the laser fluence is increased untill 0.21 J cm-2.


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a)

b)8E-09

Current (A)

6E-09

Current (A)

5E-04

Untreated

Untreated Untreated

8E-09

0E+00

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2E-07 -2E-09

-3

-1

1

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5

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c)

Photoresponse Photoresponse current DarkDark current

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Transmission (%)

Iph/Id

70

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65

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1E-05 1E-05

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2E-08 2E-07 -2E-08 2E-07

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0.19 JDark J/cm 0.19 cm2-2 UV light

-2E-09 -2E-07

700

Dark

0.19 J/cm2

Current (A) Current(A)

-5E-04 -5

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0.17 J/cm -2 0.17 J cm

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UV light Dark Dark UV light UV light

-2E-08 2E-07 5E-04 -2E-07 -2E-07 5E-04

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0E+00 0E+00 Dark Dark Dark UVUV light light UV light

Wavelength (nm)

0,1 0,1 0 0

1E-09 1E-09

0.170.17

0.190.19

0.210.21

2) -2)2) Laser fluence (J(J/cm cm Laser Fluence Laser Fluence (J/cm

-2E-07 -5E-04 -5E-04 -5 -5

-3 -3

-1 -1

1 1

3 3

5 5

Voltage (V) Voltage (V)

Figure 4. a) Dark Current vs voltage curves for untreated and samples sintered at 0.17 J cm-2, 0.19 J cm-2 and 0.21 J/.cm-2 laser fluences, Inset is the IV curve of untreated and treated sample. b) Current vs voltage curves under UV illumination for untreated and samples sintered at 0.17 J cm-2, 0.19 J cm-2and 0.21 J cm-2 laser fluences, c) Photoresponse and dark current as function of the laser fluence. Inset: optical transmission of the untreated sample and samples treated at 0.17 J cm-2and 0.19 J/.cm-2 in the 350 nm – 750 nm wavelength range.

This result can be attributed to several competing parameters which determine the photoresponse efficiencies of the investigated samples such as: (i) good semiconducting properties of the film, with additional contribution of oxygen-vacancy defect electronic states and (ii) the rough structure of the TiO2 film that ensures high number of surface trap states, thus prolonging the lifetime of photogenerated carriers.


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Indeed, the highest value of photocurrent was obtained for a sintering fluence of 0.19 J cm-2. This is due to the removal of the organic binder inducing the interconnection of the TiO2 nanoparticles which is consistent in the SEM picture from Figure 3b. Formation of necking between individual nanoparticles provides more fluent electron transfer paths across the surface of the film. Moreover, screen-printed film with TiO2 nanoparticles still has a rough film which increases light scattering effects, and thus improves the light harvesting efficiencies. If the film is treated with a laser fluence lower than 0.19 J cm-2, the sintering process increases the porosity and roughness of the film compared with the untreated sample. However, the TiO2 film still retains the organic binder, and lack of interconnection between the TiO2 particles resulting to a lower photoresponse. In the opposite case, if the film is treated with a laser fluence higher than 0.19 J cm-2, the film becomes more compact and bulky with low porosity and roughness. This morphological change in the film surface induces a better electrical conductivity but also low photoresponse due to a degradation of the active surface area. Thus, there is a competition between increasing conductivity but decreasing the active surface area with high laser fluence. Then, to improve the photoresponse of the UV sensor there is an intermediate laser fluence found in our case at 0.19 J cm-2. Moreover, the optical transmission shown in the inset of Figure 4c, measured for the untreated and samples with highest photoresponse (0.17 J cm-2 and 0.19 J cm-2), in the 350 nm – 750 nm wavelength range, support the obtained results. In fact, measurements reveal significant drop in transmission of the incident light for the laser treated samples. This drop originates from diffuse light scattering induced by surface roughness reorganization and it was noticed as change in colorization of the samples from white to black. Measured transmission spectra for all


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investigated samples have strong optical absorption at the middle of UVA (315nm – 400nm) part of the spectrum, which can be attributed to absorption of PET substrate. Optical absorption onset for laser treated sensor is slightly shifted towards visible range due to increased concentration of oxygen vacancies and defect electronic states in the band gap of titania

37

, giving rise to better

photoresponse and high photocurrent values in comparison with untreated sensor. Next, the stability of the UV sensor based on TiO2 sintered with 0.19 J cm-2 laser fluence was evaluated by measuring its time-dependent responses (photocurrent to dark current ratio) under a bias of 5 V with periodically repeated a 365-nm UV illumination (alternating dark and light conditions) with an on/off internal of 5 min. Four repeat cycles under an on/off light intensity of 20 mW cm-2are displayed in Figure 5a, in which the UV response was observed to be consistent and repeatable and this over the weeks as it is shown in Figure S4. Next, the time response of the UV sensors was estimated. The rise time (τr) and decay time (τd) are defined as the rising time ranging from 10% to 90% and the falling time ranging from 90% to 10% of the maximum photocurrent. Figure 5b shows the more detailed transient photoresponse of the device from Figure 5a. The photoresponse rise to 90% of the peak value in 25 s, while it takes longer time (50 s) to recover the initial value with turning off the UV light due to the decay process in which photo generated electrons recombine with holes in the valence band.


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a) 30

UV on

UV on

UV on

b) 30

UV on

25

25

20

Photoresponse

Photoresponse

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10

Decay time: r =50 s

20 15 10

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5

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Figure 5. a) Time-resolved of photoresponses of the TiO2 device under a continuous UV light rectangle pulse b) Highlights of the dynamic response behavior (response/recovery times) for the photodetector under UV illumination

3.3. Ethanol sensing performance In addition to UV sensors, the same screen printed TiO2 film can be improved in order to be used as ethanol sensors by simply adjusting the laser fluence of the post-treatment process. Indeed, previous studies have shown that TiO2 is appealing material for ethanol sensing, due to its wide band-gap semiconductor properties and high surface reactivity to gases. Various methods have been explored to optimize ethanol sensing properties of TiO2 such as the use of nanostructured TiO2 like nanowires, nanotubes, or/and by doping it with noble metals like silver, iron, platinum.

42-45

Moreover, all these examples of TiO2 based ethanol sensors are either

working at temperature above 150°C or/and require high calcination temperature which limits their integration into flexible substrates. Here, we propose a robust solution for producing ethanol sensors based on TiO2 nanoparticles printed on flexible substrate, operating at room temperature. The pulsed-laser treatment is introduced for the first time to replace calcination step and improve ethanol sensing properties of TiO2 by controlling the morphology and structure of TiO2 screen-printed film.


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To investigate the influence of the laser treatment on the response of the devices, the sensor response and recovery as a function of time towards 100 ppm ethanol were measured at room temperature (25°C) for untreated and treated samples. Here, the gas sensor response is given by the ratio of the initial electrical resistance in air to the electrical resistance when ethanol vapor is introduced (R0/Rg). Figure 6a clearly demonstrates that the highest response for 100 ppm ethanol vapor was measured for the sample treated at 0.17 mJ cm-2, then the response decreases as the laser fluence increases. This trend is confirmed by the Figure 6b exhibiting the sensor response for all samples as function of the ethanol concentration in which it can be noticed that by increasing the ethanol concentration, the response value increases for all samples unless for that of treated with the higher laser fluence where no significant change can be observed. Moreover, the highest response was also measured for the sample treated with the lowest laser fluence for all ethanol concentration. 50

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Figure 6. a) Time-resolved of ethanol responses of untreated and treated TiO2 film at 100 ppm. b) Gas response of untreated and laser treated TiO2 films as function of ethanol concentration The rise of the response to ethanol of the sample treated at laser power of 0.17 J cm-2 compared to the untreated sample is due to its higher porosity and roughness and higher conductivity as evidenced from the SEM images from Figure 3a and electrical characterization from Figure 4a.


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Then, by further increasing the laser fluence to 0.19 J cm-2 the response decreases while the conductivity increases and the film still maintains its porous and rough surface but to a lesser extent. At laser fluence of 0.21 J cm-2, the sensor response is almost null due to the complete loss of surface porosity, roughness and formation of bulky and dense film with lower surface-to-volume ratio, which is consistent with the SEM ans AFM observation from Figure 3 a and b. In this case, laser treatment produces rise of the conductivity, but also leads to reduction of the sensing area which is directly related to the detected drop of the sensor response. Here, it is interesting to notice that improvement of the film conductivity does not have a significant influence on the gas sensitivity of the fabricated sensors unlike the UV detectors where sustainable improvement of the UV sensitivity is achieved through compromise between the surface morphology of the film and its electrical conductivity. Indeed, the key structural parameters to determine the gas-sensing properties are the amount of activated adsorption sites and the gas-diffusion ability of the sensitive film. A rough surface with small grain size will create a higher specific surface area than a compact layer with sintered nanoparticles. Moreover, an open and porous structure will allow the diffusion and distribution of gas molecules into the volume of the film, where the inner grains can contribute to the improvement of gas-sensing properties. These results clearly demonstrate the capacity of the laser radiation to interact with the surface structures and morphologies of the MOx-based nanomaterials and to fine tune their multifunctionality. Afterwards, to confirm the potential of the fabricated sensor to be used as ethanol sensor, repeatability which is an important parameter in term of reliability of sensors, was evaluated for the sample treated with the optimized laser fluence of 0.17 J cm-2. Figure 7a illustrates the gas


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response evolutions of the laser treated sample in 5 cycles toward 100 ppm ethanol at room temperature. The reproducibility of the sensor reveals that it maintains its initial response amplitude without a clear decrease upon 5 cycle’s successive sensing tests at 100 ppm of ethanol, which is confirmed by the measurements performed for 5 weeks shown in Figure S4. Selectivity is another important aspect of the gas sensing performance. Thereupon, to explore the selectivity of the TiO2 sensors, other volatile organic pollutants (VOPs), including acetone, isopropanol and methanol, were also measured under the same conditions and the results are shown in Figure 7b. It can be seen that among all the different targeted gases tested with the same concentration of 100 ppm, the response of the TiO2 sensor to ethanol is the highest, indicating that the TiO2 based sensor has comparatively favorable selectivity to ethanol. Moreover, a fast response and recovery time are also commonly desired characters. Herein, the response time is the time taken by a sensor to achieve 90% of the maximum response, and the recovery time is the time needed for the sensor to drop to 10% of its initial response. Inset in Figure 7b shows the more detailed sensor response and recovery of this device exhibits a fast response and the recovery time of less than 1 min.

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100 ppm 100 ppm 100 ppm 100 ppm 100 ppm

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< 1s 25

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Figure 7. a) Sensor response of the sample treated at 0.17 J cm-2 under dynamic cycles between 0 ppm and 100 ppm concentration of ethanol at 25°C. b) Response characteristics of the TiO2


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sensor, measured at room temperature for various volatile organic poluants under same concentration of 100 ppm. Inset: Highlight of the dynamic response behavior (response/recovery times) for the sensor under a ethanol concentration of 100 ppm

3.4. Flexibility performance Mechanical stability is essential to flexible electronic devices, especially for applications where high stability over the mechanical deformation is required, such as wearable electronics and smart food packaging. Therefore, the influence of the mechanical strain on the sensor response behavior has to be investigated. Sensing measurements were then performed when the sensors were bended at 40° and after returning to the flat position. The optical image of the screenprinted TiO2 sensor under bending at 40° is shown in Figure 8a. The schematic image of the direction of the bending-induced mechanical strain in the sensor is described in Figure 8b. Figure 8c shows the real-time photoresponse of the sensor sintered at 0.19 J cm-2 under 365nm light illumination (20 mW cm-2) at room-temperature when the sensor is in its initial position, bended at 40° and after its return to the flat position. From the obtained results it can be concluded that the sensors’ response showed negligible effect over the mechanical strain. The same measurements were also performed at room temperature for the device sintered at 0.17 J cm-2 at 100 ppm ethanol concentration as exhibited in Figure 8d. This figure clearly demonstrates that, as in the previous case, the effect of the mechanical strain can be neglected. Thus, these results clearly indicate that bending of the sensor did not affect the sensing performance of the fabricated devices.


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Figure 8. a) Optical image of the fabricated flexible TiO2-based sensor. b) The schematic image of the direction of the bending of the sensor, and the corresponding curvature angle. c) Timeresolved of UV sensor responses of 0.19 J cm-2 treated TiO2 film under UV light and bending condion. d) Time-resolved of ethanol responses of 0.17 J cm-2 treated TiO2 film at 100 ppm under bending condition.

4. CONCLUSION In summary, we have developed a new strategy for producing low‐ cost, scalable, flexible and multifunctional devices for UV detection and ethanol sensing, which is based on printed TiO2 nanoparticles with laser-tunable properties. Main idea behind undertaken approach is the association of two different techniques: screen-printing and laser treatment process. The first technique is coherent for the patterning of multisensing devices which consist of TiO2 active


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layer deposited on the top of silver interdigitated electrodes. The second one is particularly adapted for the post treatment of the active layers on flexible substrates and allows to perform simultaneously and selectively the sintering of TiO2 patterned films in a single step. Both approaches are fast, cost-effective and do not require annealing steps at high temperatures, which makes them compatible with any kind of flexible substrates. The effects of the laser fluence on morphology, electrical properties, UV sensing and gas sensing properties are disclosed. The UV photoresponse and ethanol response were improved by properly adjusting the laser fluence. It was found that this phenomenon is correlated to the change of the morphology and structure of the TiO2 film generated by the laser irradiation. In addition, the resulting UV and ethanol sensors showed good repeatability, relatively fast response time and good mechanical flexibility. Therefore, the possibility to fabricate multifunctional devices with tunable properties, in a rapid and large-scale manner, paves the way to low-cost solutions of sophisticated multisensing technologies, printed on flexible substrates. Supporting Information. Multi profile analysis of the SEM images from Figure 3a of untreated sample and samples treated at 0.17, 0.19 and 0.21 J cm-2; Roughness measurements obtained from the multi profile analysis; Energy Dispersive X-Ray analysis of the untreated and treated at 0.21 J cm-2 TiO2 film; Ethanol (100 ppm) and UV response (20 mW cm-2) of the devices over the weeks.

AUTHOR INFORMATION Corresponding Author *Georges Dubourg


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E-mail: [email protected] Author Contributions G.D. conceived the idea, designed and performed the experiments. M.R. contributed in material preparation (TiO2 paste) and optical and structural characterization; G.D. and M.R. analyzed the data. G.D wrote the manuscript. Funding Sources This work is supported by the Serbian Ministry of Education, Science and Technological Development through the project III44006 and OI171032.. Acknowledgment The authors would like to acknowledge Dr Borislav Vasić for AFM measurements. Note The authors declare no competing financial interest

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Table of Contents Graphic

Ethanol sensor H

UV detector

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Multifunctional Screen-Printed TiO2 Nanoparticles ... - ACS Publications

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